1. Field of the Invention
The present invention relates to a method of polishing a glass substrate for information recording media, and a glass substrate for information recording media.
2. Description of the Related Art
Conventionally, glass substrates for information recording media used in hard disk drives (hereinafter referred to as xe2x80x9cHDDsxe2x80x9d) or the like have been manufactured through a series of steps such as the following.
1. Disk processing step in which a glass plate is processed into a donut-shaped glass substrate.
2. Chamfering step in which chamfered surfaces are formed at edge parts at the inner and outer peripheries of the donut-shaped glass substrate.
3. EP (edge polishing) step in which the edge surfaces at the inner and outer peripheries of the glass substrate are polished.
4. Surface polishing step in which the major surfaces of the glass substrate to form recording surfaces are polished.
5. Chemical strengthening step in which sodium ions and potassium ions are introduced into the surfaces of the glass substrate, thus strengthening the glass substrate.
6. Inspection step in which the glass substrate that has been subjected to all of the steps up to the chemical strengthening step is inspected with regard to whether or not certain predetermined criteria are satisfied.
The surface polishing step, which is the fourth step, is comprised of a lapping step for reducing the thickness of the glass substrate to a certain predetermined value, and a precision polishing step for giving the major surfaces of the glass substrate a precision finish.
In the lapping step, rough grinding is carried out using abrasive grains of alumina or the like until the thickness of the glass substrate becomes the predetermined value. The precision polishing step following the lapping step is comprised of a first polishing step and a second polishing step. In the first polishing step, the major surfaces of the glass substrate are polished using abrasive grains of cerium oxide or the like, to remove minute cracks in the major surfaces that have arisen through the lapping step and to make the major surfaces into mirror surfaces. In the second polishing step, the major surfaces are polished using abrasive grains having a mean grain diameter lower than the mean grain diameter of the abrasive grains used in the first polishing step, thus finishing the major surfaces.
Of these steps in the surface polishing step, the product quality of the glass substrate is affected in particular by the first polishing step and the second polishing step. Here, the product quality of the glass substrate relates to the shape of the edge parts of the glass substrate, the roughness of the surfaces, and the minute waviness of the surfaces.
FIG. 6 is a sectioned perspective view of a glass substrate. FIGS. 7A and 7B are sectional views showing the cross-sectional shape around an edge part of a glass substrate; FIG. 7A shows the edge part shape in the case that roll-off has not occurred, and FIG. 7B shows the edge part shape in the case that roll-off has occurred.
The product quality with regard to the edge part shape of the glass substrate refers to the extent of so-called xe2x80x9croll-offxe2x80x9d in which a peripheral part of the recording surface is shaved off more than a central part thereof (see FIG. 7B). If the extent of roll-off becomes larger, then the difference in the so-called xe2x80x9cflying heightxe2x80x9d, which is the distance from the recording surface to the magnetic head, between the central part and the peripheral part of the recording surface becomes larger, and hence the traveling stability of the magnetic head and the accuracy of reading and writing drop, and thus errors occur more frequently. Consequently, the lower the extent of roll-off, the better the product quality.
The product quality with regard to the roughness of the surfaces of the glass substrate refers to the height of projections in particular out of undulations formed on the glass substrate surfaces due to compressive stress generated during the precision polishing step. If the projections are high, then head crashes may occur in which the magnetic head of the HDD collides with the projections, or thermal asperity may occur in which heat generated through such collisions causes malfunctioning in which the magnetic head detects abnormal signals. Consequently, the lower the projections, the better the product quality with regard to the roughness.
The product quality with regard to the minute waviness of the surfaces of the glass substrate refers to the form of appearance of undulations larger than the xe2x80x9croughnessxe2x80x9d.
The following is an explanation of the minute waviness. The xe2x80x9cminute wavinessxe2x80x9d is one aspect of the shape of a substrate surface, and refers to a wave-like shape of wavelength of the order of several hundred microns to millimeters and amplitude of the order of nanometers. Undulations of wavelength shorter than this constitute the xe2x80x9croughnessxe2x80x9d described above, whereas undulations of wavelength longer than this come under xe2x80x9cflatnessxe2x80x9d. There are no precise criteria for classifying into xe2x80x9croughnessxe2x80x9d, xe2x80x9cminute wavinessxe2x80x9d and xe2x80x9cflatnessxe2x80x9d. On an actual glass substrate surface, undulations having a wavelength and an amplitude both of the order of nanometers (hereinafter referred to as xe2x80x9cultra-small undulationsxe2x80x9d) are present in random fashion.
The form of appearance of the ultra-small undulations as captured at a span of the order of nanometers is the xe2x80x9croughnessxe2x80x9d. Within the roughness, the form of appearance of the ultra-small undulations is random, but if one looks with a relatively long span, then a regularity to the wavelength can be seen. This regularity of the wavelength in the form of appearance of the ultra-small undulations corresponds to the xe2x80x9cminute wavinessxe2x80x9d. The minute waviness can be measured using an Optiflat (product name; made by Phase Shift Technology) optical measuring apparatus. To increase the recording density of a magnetic recording medium, it is necessary to reduce the flying height, and hence it is necessary to make the minute waviness small. Consequently, the smaller the minute waviness, the better the product quality.
In recent years, with increases in the recording density of magnetic recording media, demands on the product quality as described above of glass substrates have become increasingly strict.
Conventionally, as shown in FIG. 8, in the precision polishing step, polishing has been carried out for a time period t1, by controlling the polishing load P1 applied to the glass substrate and the rotational speed R1 (the number of revolutions per unit time) of the plates on which the polishing members that polish the glass substrate are mounted. To meet the required product quality as described above, the polishing has been carried out with the polishing load P1 reduced and the rotational speed R1 of the plates reduced.
However, with such a glass substrate polishing method, there is a problem that the polishing rate (the polishing amount per unit time) drops, and hence the time t1 required for the precision polishing step becomes long, and thus the productivity becomes poor.
It is an object of the present invention to provide a method of polishing a glass substrate for information recording media, and a glass substrate for information recording media, which enable polishing to be carried out such that the product quality is improved, without the productivity dropping, and without the processing cost increasing.
To attain the above object, the present invention provides a method of polishing a glass substrate for information recording media, comprising a precision polishing step having a first polishing step and a second polishing step carried out in this order, in which, after carrying out rough polishing on major surfaces of a glass substrate, precision polishing is carried out on the major surfaces by feeding abrasive grains onto the major surfaces, pushing polishing members against the major surfaces, and rotating the major surfaces and the polishing members relative to one another, wherein at least one of the first polishing step and the second polishing step has a preceding polishing step and a succeeding polishing step carried out in this order, and a mean grain diameter of the abrasive grains in the succeeding polishing step is not more than in the preceding polishing step, and a pressure at which the polishing members are pushed against the major surfaces is lower in the succeeding polishing step than in the preceding polishing step.
According to the method of polishing a glass substrate for information recording media of the present invention, the precision polishing step has a first polishing step and a second polishing step carried out in this order, and at least one of the first and second polishing steps has a preceding polishing step and a succeeding polishing step carried out in this order, wherein the mean grain diameter of the abrasive grains in the succeeding polishing step is not more than in the preceding polishing step, and the pressure at which the polishing members are pushed against the major surfaces is lower in the succeeding polishing step than in the preceding polishing step. As a result, the major surfaces of the glass substrate can be polished rapidly until the desired product quality is nearly reached through the preceding polishing step, and then the major surfaces can be finished to a state in which the desired product quality is satisfied through the succeeding polishing step. The polishing can thus be carried out such that the product quality of the glass substrate for information recording media is improved, without the productivity dropping, and without the processing cost increasing.
In a first preferred form of the present invention, the second polishing step has a preceding polishing step and a succeeding polishing step carried out in this order, and a mean grain diameter of the abrasive grains in the succeeding polishing step is not more than in the preceding polishing step, and a pressure at which the polishing members are pushed against the major surfaces is lower in the succeeding polishing step than in the preceding polishing step.
Moreover, it is preferable that the succeeding polishing step has a plurality of steps each having a different combination of the mean grain diameter of the abrasive grains and the pressure at which the polishing members are pushed against the major surfaces. As a result, the optimum number of steps and the optimum combinations of the mean grain diameter of the abrasive grains and the pressure can be set in accordance with the material properties and so on of the glass substrate for information recording media. The polishing can thus be carried out in accordance with the material properties and so on of the glass substrate for information recording media, such that the productivity and the product quality are further improved, without the processing cost increasing.
Preferably, the pressure at which the polishing members are pushed against the major surfaces is in a range of 80 to 200 g/cm2 in the first polishing step, in a range of 80 to 150 g/cm2 in the preceding polishing step, and in a range of 30 to 70 g/cm2 in the succeeding polishing step.
In a second preferred form of the present invention, the precision polishing step has a first polishing step and a second polishing step carried out in this order, and the first polishing step has a preceding polishing step and a succeeding polishing step carried out in this order, wherein the mean grain diameter of the abrasive grains in the succeeding polishing step is not more than in the preceding polishing step, and the pressure at which the polishing members are pushed against the major surfaces is lower in the succeeding polishing step than in the preceding polishing step.
In the second preferred form, it is preferable that the succeeding polishing step has a plurality of steps each having a different combination of the mean grain diameter of the abrasive grains and the pressure at which the polishing members are pushed against the major surfaces.
In the second preferred form, it as preferable that the pressure at which the polishing members are pushed against the major surfaces is in a range of 90 to 200 g/cm2 in the preceding polishing step, in a range of 30 to 80 g/cm2 in the succeeding polishing step, and in a range of 30 to 80 g/cm2 in the second polishing step.
In a third preferred form of the present invention, the precision polishing step has a first polishing step and a second polishing step carried out in this order, the first polishing step has a preceding polishing step and a succeeding polishing step carried out in this order, wherein the mean grain diameter of the abrasive grains in the succeeding polishing step is not more than in the preceding polishing step, and the pressure at which the polishing members are pushed against the major surfaces is lower in the succeeding polishing step than in the preceding polishing step, and the second polishing step has a preceding polishing step and a succeeding polishing step carried out in this order, wherein the mean grain diameter of the abrasive grains in the succeeding polishing step is not more than in the preceding polishing step, and the pressure at which the polishing members are pushed against the major surfaces is lower in the succeeding polishing step than in the preceding polishing step.
In the third preferred form, it is preferable that the succeeding polishing step of the first polishing step has a plurality of steps each having a different combination of the mean grain diameter of the abrasive grains and the pressure at which the polishing members are pushed against the major surfaces, and the succeeding polishing step of the second polishing step has a plurality of steps each having a different combination of the mean grain diameter of the abrasive grains and the pressure at which the polishing members are pushed against the major surfaces.
In the third preferred form, it is preferable that the pressure at which the polishing members are pushed against the major surfaces is in a range of 90 to 200 g/cm2 in the preceding polishing step of the first polishing step, in a range of 30 to 80 g/cm2 in the succeeding polishing step of the first polishing step, in a range of 80 to 150 g/cm2 in the preceding polishing step of the second polishing step, and in a range of 30 to 70 g/cm2 in the succeeding polishing step of the second polishing step.
It is preferable that the polishing members are hard urethane pads in the first polishing step, and soft suede pads in the second polishing step.
It is also preferable that the abrasive grains in the first polishing step are cerium oxide abrasive grains having a mean grain diameter in a range of 1.1 to 1.5 xcexcm, and the abrasive grains in the second polishing step are cerium oxide abrasive grains having a mean grain diameter of not more than 1 xcexcm.
It is preferable that a polishing amount of the glass substrate in the second polishing step is not less than 3 xcexcm.
It is preferable that a polishing amount of the glass substrate in the first polishing step is not less than 30 xcexcm.
It is preferable that a total polishing amount of the glass substrate through the first polishing step and the second polishing step is in a range of 33 to 100 xcexcm.
To attain the above object, there is also provided a glass substrate for information recording media obtained by polishing using a method of polishing a glass substrate for information recording media according to the present invention, whereby substantially the same results as obtained with the method according to the present invention can be provided.
The above and other objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.